Selective defoliation of a field growing crop such as grape vines has long been known as a desirable practice. Thus, for example, U.S. Pat. No. 2,865,135, issued Dec. 23, 1958, inventors Gamboni et al. discloses a grape leaf stripping mechanism to strip the leaves from grape vines by a mechanical apparatus rather than manual defoliation. The apparatus disclosed by this patent uses a pair of beaters to effect the defoliation.
More recently, the University of California publication on Grape Pest Management (2nd Edition, 1992, publication 3343) notes that among the practices for disease control are the removal of basal leaves from vines approximately two weeks after bloom. This practice reduces Botrytis bunch rot and produces a superior wine grape in many North Coast vineyards, and has also been reported to reduce first-generation leaf hoppers. The publication notes that most leaf removal is presently done by hand, but that mechanical systems are being developed and used by several growers. (Supra, xi.)
Removal of basal leaves on grape vines also permits the grape clusters to hang free and allows light penetration for coloring and budwood development, and allows air and chemical spray penetration for reducing molds, moisture, and insect infestation. Leaf removal has proven to produce a better acid/sugar ratio in ripening berries, and thus results in better quality wines and/or juices.
Manual defoliation is, obviously, labor intensive and thus tends to be costly. In addition, manual defoliation when performed shortly after fruiting tends to bruise the very small berries in the fruit cluster. Further, manual defoliation tends simply to displace pests such as leaf hoppers to the adjacent, remaining leaves where they can continue to wreak their damage and subsequently can return to the fruit cluster.
The presently known mechanical means for defoliating are disadvantageous because those that blow air or pull a vacuum tend to damage the berries by bruising or removing the berries themselves.
Another problem area for cultivated fields and crop maintenance such as vineyards particularly in California are that typical soils contain nematodes that feed on roots, which reduce root efficiency. Nematode infestation of vineyards are manifested by reduced vigor and yield with slight yellowing of leaves because nematode-infected vine roots are unable to meet above-ground demands for nutrients and water.
Preplant fumigants such as methyl bromide profoundly affect nematodes, and current advice for preparing a new vineyard includes the application of a nematicide such as methyl bromide (Grape Pest Management, supra, p. 290). However, methyl bromide has been implicated in harming the ozone layer. As a consequence, considerable environmental pressure has been building against its use. Nevertheless, in February 1996, the California Senate voted to delay a proposed ban on the ozone-depleting fumigant, methyl bromide, until the end of 1997. A reason for delaying the proposed ban is due to lack of effective alternatives for agriculture in soil pest control, particularly for crops such as strawberries.
On small scale soil treatments, nurseries have been able to use steam as a sterilant for minimizing fungal infections. Thus, bedding plant nurseries have been able to grow seedlings in steam sterilized soils. But such small scale soil treatments are not presently practical or possible for in the field use. Further, the steam used is not a very efficient process, since it involves for example, temperatures close to 212.degree. F.